Add onboarding wizard for teaching the record key (#301)

* Add first-run onboarding wizard for recording mechanics

Introduce a minimal, skippable first-run wizard that teaches the record
key. On first authenticated launch it renders as a full-screen takeover
(gated above PusherProvider) so the stream/compose keyboard handlers are
not mounted to compete for the same keys.

Steps: welcome, hold ` to record, tap V to switch audio/video, tap ` to
toggle recording, mic/camera permission priming, and a recap. The lessons
detect the real gestures and never block — Esc or the Skip control
dismisses at any point.

Reuses existing pieces: VideoAudioToggle, useMediaDevices().requestLabels
for permission priming, KeyHint, and isTypingTarget. Extracts the shared
HOLD_THRESHOLD_MS into lib/constants and reuses it in the compose overlay.

Resolves #259

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01A9AHySdLb1zfZAEm6SuaQP

* nits

* light mode support for onboarding

---------

Co-authored-by: Claude <[email protected]>

js/desktop/src/features/compose/compose-overlay.tsx

@@ -29,7 +29,11 @@ import { useMediaDevices } from '@/hooks/use-media-devices';
 import { resolveEffectiveDeviceId } from '@/hooks/use-effective-device-id';
 import { useFileInput } from '@/hooks/use-file-input';
 import { createImageThumbnail } from '@/lib/image-thumbnail';
-import { MAX_ATTACHMENT_SIZE_BYTES, MAX_ATTACHMENTS } from '@/lib/constants';
+import {
+  HOLD_THRESHOLD_MS,
+  MAX_ATTACHMENT_SIZE_BYTES,
+  MAX_ATTACHMENTS,
+} from '@/lib/constants';
 import type { PendingAttachment } from '@/features/compose/attachment-strip';
 import { useSuspendPlayback } from '@/hooks/use-suspend-playback';
 import { useComposeIntentStore } from '@/stores/compose-intent-store';
@@ -59,8 +63,6 @@ interface ComposeOverlayProps {
   onParticleCreated?: (particleId: string) => void;
 }
 
-const HOLD_THRESHOLD_MS = 250;
-
 /**
  * Self-contained compose overlay. Each consumer renders its own instance
  * with props that determine the mode (new stream vs. reply).